Make your way to the North-East of Taverley and speak to the Wise Old Man. He will say that a disturbance has caused Taverley to split open. Now, go over and inspect the rip in the ground just north of the Wise Old Man. When you inspect the rip you will gain a smelted rock. Bring the smelted rock back to the Wise Old Man, he will identify it as a Rune that belonged to the mighty Titan: Poseon. Terrified, the Wise Old Man will then tell the tales of the Titans. After the cutscene, he will ask if you're ready to go with him into the Temple of Water. When you're ready, click the option "Yes".